It is known that various dissolved materials which are naturally or synthetically occurring in aqueous systems, especially aqueous systems using water derived from natural resources such as seawater, rivers, lakes and the like, attack metals. Typical aqueous systems having metal parts which are subject to corrosion include circulating water systems such as evaporators, single and multi-pass heat exchangers, cooling towers, and associated equipment and the like. As the circulating water passes through or over the system, a portion of the system water evaporates thereby increasing the concentration of the dissolved materials contained in the system. These materials approach and reach a concentration at which they may cause severe pitting and corrosion which eventually requires replacement of the metal parts. Various corrosion inhibitors have been previously used to treat these systems.
For example, chromates, inorganic phosphates and/or polyphosphates have been used to inhibit the corrosion of metals which are in contact with water. The chromates, though effective, are highly toxic and consequently present handling and disposal problems. While phosphates are non-toxic, due to the limited solubility of calcium phosphate, it is difficult to maintain adequate concentrations of phosphates in many aqueous systems. Polyphosphates are also relatively non-toxic, but tend to hydrolyze to form orthophosphate which in turn, like phosphate itself, can create scale and sludge problems in aqueous systems (e.g. by combining with calcium in the system to form calcium phosphate). Moreover, where there is concern over eutrophication of receiving waters, excess phosphate compounds can serve as nutrient sources. Borates, nitrates, and nitrites have also been used for corrosion inhibition. These too can serve as nutrients in low concentrations, and/or represent potential health concerns at high concentrations.
Environmental considerations have also recently increased concerns over the discharge of metal corrosion inhibitors such as zinc, which previously were considered acceptable for water treatment.
Much recent research has concerned development of organic corrosion inhibitors which can reduce reliance on the traditional inorganic inhibitors. Among the organic inhibitors successfully employed are organic phosphonates. These compounds may generally be used without detrimentally interfering with other conventional water treatment additives. However, environmental concerns about the discharge of phosphorus in the form of organic phosphonates have begun to be heard. It is anticipated that in the future this will lead to limitations on the use of organic phosphonates in water treatment.
Another serious problem in industrial aqueous systems, especially in cooling water systems, evaporators, and boilers is the deposition onto heat transfer surfaces of scale, particularly scale-forming salts such as certain carbonates, hydroxides, silicates and sulfates of cations such as calcium and magnesium. These systems contain relatively high concentrations of calcium carbonate, calcium sulfate and other hardness salts. Because of the evaporation which takes place in these aqueous systems, these salts in the water become more concentrated. Many organic corrosion inhibitors (e.g. hydroxyethylidene diphosphonic acid) are very sensitive to calcium i.e., they have a high tendency to precipitate with calcium ions in solution and are thus rendered ineffective.
Thus, there is a continuing need for safe and effective water treating agents which can be used to control corrosion, particularly when a substantial concentration of dissolved calcium is present in the system water. Water treating agents of this type are particularly advantageous when they are phosphorus-free.